1. Field of the Invention
The present invention relates to hydrogen-absorbing alloy electrodes for use as negative electrodes in alkaline storage batteries. The invention also relates to alkaline storage batteries and methods of manufacturing the alkaline storage batteries. More particularly, the invention relates to an improved hydrogen-absorbing alloy electrode for use as a negative electrode in an alkaline storage battery for improving the low temperature discharge capability and high rate discharge capability of the alkaline storage battery.
2. Description of Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel-metal hydride storage batteries employing a hydrogen-absorbing alloy as a material for their negative electrodes have drawn considerable attention in that they have higher capacity than nickel-cadmium storage batteries and that, being free of cadmium, they are more environmentally safe.
As alkaline storage batteries comprising the nickel-metal hydride storage batteries are used in various portable devices, demands for further higher performance in the alkaline storage batteries have been increasing.
In the alkaline storage batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni have been commonly used as the negative electrodes.
However, these hydrogen-absorbing alloys do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to further enhance the capacity of the alkaline storage batteries.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloys, it has been proposed to use a hydrogen-absorbing alloy having a Ce2Ni7 type or a CeNi3 type crystal structure, other than the CaCu5 type, by adding Mg or the like to the rare earth-nickel hydrogen-absorbing alloy. (See, for example, Japanese Unexamined Patent Publication No. 2002-69554.)
Nevertheless, the alkaline storage battery that uses as the negative electrode a hydrogen-absorbing alloy electrode employing a hydrogen-absorbing alloy in which Mg or the like is added to the rare earth-nickel hydrogen absorbing alloy still has the problems of poorer high rate discharge capability and low temperature discharge capability than those of nickel-cadmium storage batteries.
On the other hand, Japanese Patent No. 3241047, for example, proposes a hydrogen storage alloy having a surface provided with enriched nickel regions of 7 nm or less in diameter so that the energy density and so forth can be improved.
Nevertheless, even with the use of the hydrogen-absorbing alloy having a surface provided with enriched nickel regions of 7 nm or less in diameter for an alkaline storage battery, electric charge does not transfer smoothly on the surface of the hydrogen-absorbing alloy, so it has been difficult to sufficiently improve high rate discharge capability and low temperature discharge capability.